1. Field of the Invention
The invention relates to a clamping device for machine tools, and provided with a power-operated chuck for holding a workpiece wherein clamping jaws can be actuated using the clamping device by means of an axially moveable draw rod, wherein the clamping device is provided with an electric servomotor with a changeover function for triggering clamping movements, a movement converter for converting adjustment movements of a rotor shaft of the servomotor into axial movements of the draw rod required for actuating the clamping jaws, as well as a force accumulator for maintaining a clamping force and comprising preloaded spring packs supported on a spindle nut of the movement converter.
2. Description of the Prior Art
A device of this kind for clamping a workpiece is disclosed in DE 10 2009 044 167 A1. In this embodiment, a spindle nut is embedded between two spring packs configured as helical compression springs, and a draw rod and the spindle nut each have a sensor in order to register their axial positions. The occurrence of a slight axial force in the form of contact between clamping jaws and the workpiece to be machine is recorded by means of a first sensor. The continued movement of the spindle nut against the force of one of the two spring packs is detected by a second sensor, and a resulting sensor differential between the two sensors is used for calculating a clamping force. The travel difference between the first sensor and the second sensor is thus measured continuously by both sensors, and is used for calculating the clamping force with the help of a control unit.
The spindle nut in the clamping device is undefinably clamped between the two spring packs inserted in adjustable bushes, in which case the spring characteristics of the spring packs cannot be the same. The sensor differential measured by the two sensors is thus subject to considerable inaccuracies, particularly since the spring forces can only apply a maximum of 50% of the particular clamping force and additional inaccuracies have to be tolerated because of unavoidable friction forces. Although the complexity of the clamping device and the control unit in particular is very considerable, unpredictable responses cannot be avoided or compensated for, with the effect that exact, position-dependent measurement of the clamping force is not possible.